The invention relates to a contact switch for an electrical appliance, which has a sensor element with a sensor surface made from flat material and is connected to a control for the sensor element and with it is associated an illuminated display with a housing and lighting means located therein.
Contact switches with rectangular, metallic sensor surfaces are known, e.g. conducting paths or surfaces on ceramic supports, which can be resiliently mounted. Such contact switches operate capacitively, which means that the approach or contact of a finger gives rise to a capacitor change on the part of the sensor element, which can be detected. They can be pressed by means of metal springs or the like onto the underside of a cover, e.g. a glass ceramic plate of a cooking area. The arrangement of such resilient sensor elements, their adaptation to a cover and fitting problems make use difficult.
The problem of the invention is to provide a contact switch, which overcomes the disadvantages of the prior art and in which it is in particular improved with respect to effectiveness and the multiplicity of uses.
According to the invention the sensor surface passes over the housing and has a cutout corresponding to the shape of the lighting means and essentially in overlap-free manner surrounds said lighting means. This makes it possible to place the sensor surface so-to-speak between the illuminated display and the actuating surface or the location of an actuation, without shielding the illuminated display.
The contact switch or sensor element can be fitted below a substantially closed cover and a translucent cover is considered to be particularly suitable in order to render the lighting means visible. One possibility is a glass ceramic plate of an electrical cooking area. Advantageously the sensor element with the sensor surface engages on the underside of the cover and is in particular pressed directly onto it with a certain force. Actuation of the contact switch takes place by means of the capacitive coupling of a finger to the sensor element through the cover, so that it is very advantageous if the distance between the top of the cover and the sensor surface is substantially always identical. This can be achieved by a pressing action.
Preferably the sensor element is substantially metallic and can e.g. comprise a thin metal sheet. In particular, the sensor surface is planar. One possibility for the production of the above-described sensor element is to work or punch it completely and in a single process from a surface, particularly a metal plate. It can in particular have a portion with which it is conductively and/or in fastening manner connected to a base plate, particularly a printed circuit board or the like. The portion is advantageously constructed as a plug-in or standing leg and can be worked from the sensor element material by bending round. According to a variant a standing leg can have a rigid construction. According to a particularly preferred variant, especially in the case of one-piece manufacture, the standing legs do not extend over the outer boundaries of the surface area of the sensor element and prior to bending round preferably run in an inner face of the sensor element.
The sensor element can substantially frame the lighting means, preferably as a closed frame. As a function of the type of illuminated display or the intended actuation of the contact switch, the frame can have a width of between 1 to 5 mm and the width in particular varies between these values.
According to a preferred embodiment the sensor element is at least partly in engagement with the housing of the illuminated display, particularly on the top of the housing facing an actuating means or cover. This is particularly advantageous in illuminated displays with a housing, in which the lighting means maintain a clear spacing from the edge towards the inside. This spacing can be utilized as a bearing surface for the sensor element. The bearing action in particular permits a mechanical stabilization of the sensor element, without this loading capacity having to be provided by its own standing legs.
According to a particularly preferred development of the invention the sensor surface can have an at least partly flexible, electrically conductive coating.
A cover for the contact switch, in the case of a glass ceramic plate in particular, can have on its underside an irregular surface, particularly a standard stud arrangement. Thus metal plates rest on the tips of the highest irregularities. This leads to a disturbing air gap and the distance between the actuating surface and the sensor surface is not constant in the case of manufacture. However, a flexible, adaptable coating is ideally applied to the non-planar underside of a cover and thus avoids the aforementioned problems. Preferably plastic or rubber materials are used, which are made electrically conductive by admixing carbon, carbon black, metal, etc. Advantageously silicones are used in areas where high temperatures are encountered. As a function of the intended use and standard unevenness, the coating thickness can vary and it is advantageous to adopt a thickness between 0.1 and 2 mm, particularly of approximately 0.8 mm. The flexible material can have a Shore hardness range between 5 and 20, particular preference being given to approximately 10 to 15 Shore.
One possibility consists of constructing the coating as a contact plate of electrically conductive, flexible expanded or foamed material. It can in particular be worked or punched from a large-area web material and is advantageously subsequently applied to the sensor surface. The application or fastening is intended to form an electrically conductive transition and this can e.g. be achieved by means of a conductive adhesive or a corresponding adhesive tape. One possibility consists of providing the contact plates in the manner of stick-on labels on large-area webs or long rolls with a slightly adhering cover film for the adhesive coating. It is always possible to remove a contact plate from said reservoir and adhere it to a sensor surface. For this purpose mechanical and automatic processes are also known and preferably used.
The coating is preferably intended to provide an electrically conductive binding to the underside of a cover, so that it is made electrically conductive. The electrical resistance of the coating from surface to surface can be within a range of a few kiloohms to 15 kiloohm and preferably with an approximately 0.8 mm thick contact plate is approximately 7 kiloohms. For the resistance it must be borne in mind that the pressure exerted during operation on the coating must be taken into account.
As lighting means preference is given to those with a substantially parallelepipedic housing with a planar top surface on or below which are located the lighting means or through which the latter emit. The lighting means are advantageously light emitting diodes. In a preferred embodiment the illuminated display is a digital display, particularly a seven-segment display, the segments preferably being formed by light emitting diodes. Another possibility is provided by LCD""s, which as a result of their lower light emission should be placed under a very transparent or translucent cover. When using background illuminated LCD""s, dark covers are also possible.
To the sensor element can be connected switching means, e.g. relays, supplied following an actuation of the sensor surface with a signal produced at the sensor element in a converted form and switch an electric power. In this way working only takes place at the sensor element with signal levels, whereas higher power levels can be separately switched with conventional switches and electrical contacts or the like.
These and further features can be gathered from the claims, description and drawings and the individual features, either singly or in the form of subcombinations, can be implemented in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions for which protection is claimed here. The subdivision of the application into individual sections and the subtitles in no way limit the general validity of the statements made thereunder.